In the intro of season 1, episode 1 of The Wire, detective McNulty and some random dude talk about the Snot Boogie being murdered because of a craps game. What's the meaning behind this intro scene and the dialog? What does "This' America, man" symbolizes?

Man On Stoop: I’m sayin’, every Friday night in an alley behind the
Cut Rate, we rollin' bones, you know? I mean all them boys, we roll
til late.

McNulty: Alley crap game, right?

Man On Stoop: Like every
time, Snot, he'd fade a few shooters, play it out til the pot's deep.
Snatch and run.

McNulty: What, every time?

Man On Stoop: Couldn’t help
hisself.

McNulty: Let me understand. Every Friday night, you and your
boys are shooting craps, right? And every Friday night, your pal Snot
Boogie… he'd wait til there's cash on the ground and he'd grab it and
run away? You let him do that?

O say does that star-spangled banner yet wave
O’er the land of the
free and the home of the brave?

This is America and everyone is free to do what the want. They let him play because this is America and he could play. And if he cheated they'd beat him up, but they'd never stop him playing cause it's America and it's his right. That's certainly one interpretation and impression I was left with.

A second and much larger one though is that it's all a game. This site have a wonderful explanation which I'll simply lift. They describe two types of people in the show, along with one overall concept:

The Streets
As writer David Simon puts it, “those who are excluded
from the legitimate economy make their own world.” This is truly
another world. We see young kids out on the stoop unsupervised, just
watching the police work as if nothing’s wrong. As if there’s not a
bloody body out in the street.

The Police
These people generally aren’t familiar with the streets.
Even when they are, their understanding is juvenile at best. Still,
they are forced to interact with this different world, which means
they often depend on those who live there. This relationship is
examined in-depth throughout this season.

The Game
This is what unites the two different worlds. The “game” has
many levels, and while it may differ on some levels, the rules are
always the same: there are winners and there and losers, and you play
the game whether you like it or not.

In other words, he might have stolen the money every time and they might have beaten him up everytime - but he was from the streets and they were from the streets. They have to play the game together and regardless of how people play, you punish them if required and then let them keep on playing.

Another example of this can be seen clearly in the first episode with Bubbles and Johnny Weeks. They get beaten up for using counterfeit money to buy their drugs. The next time they try the scam, Johnny is beaten up. This doesn't mean Bubbles can no longer show his face in the neighbourhood or anything like that. They're all still playing the game. They got their punishment, and now they can carry on playing, purchasing their drugs and surviving.

It can also be argued (and has been in several online boards) that the line demonstrates a parallel between Snot Boogie's death and the failure of the State in the drug war, with disproportionate amounts of violence being rendered against the powerless (and indeed Snot Boogie is just a teenager) and no real progress being made despite time dragging on and on. In Snot Boogie's case, he was the victim of violence far beyond what he caused, in a dance (of theft) that carried on and on with no real reason other than it could.

A blog of one of the New Jersey Local News discusses this as well, and as it phrases it so eloquently, I'll simply quote directly from it:

David Simon likes to say that the first scene of each season of "The
Wire" encapsulates the themes of that season. In the case of Detective
Jimmy McNulty investigating the murder of one Omar Isiah Betts, known
to friends and family as Snot Boogie, Simon gets to explain what the
entire series will be about...

...The America of "The Wire" is broken, in a fundamental, probably
irreparable way. It is an interconnected network of ossified
institutions, all of them so committed to perpetuating their own
business-as-usual approach, that they keep letting their own
equivalents of Snot Boogie into the game, simply because that's how
it's always been done. It doesn't matter that it makes no sense. Only
a rugged individualist/cocky narcissist like McNulty would even think
to suggest that things could and should be run differently.

So there's a whole collection of different interpretations. To me, the strongest of them has always been the idea of people playing the game, not out of choice, but due to the hand that life dealt them.

Another interpretation I always got of this, and it's similar to the "this is America, land of the free, and you can do whatever you want" approach. This is also America, where you are innocent until proven guilty.
I.e., Snotboogie always "commits the crime" of stealing the money from the craps game. And every week he gets his punishment of getting his ass kicked because of it. After that, he's innocent. And even if everyone is absolutely 100% positive that he'll probably do the same thing next week, they have to let him in the game.

What I got from this particular line is that, regardless of how of their gangster ways they hold “rights” and “freedom” in high regard. The rules are the rules. There was a scene in a latter episode where Avon was talking about no shooting the Sunday truce. This further reiterates that fact.

I think it summarizes the fact that America has free association, not just that "people can do what they want", but that we don't exclude very much within our communities. I was surprised that in Europe you often have to specifically invite everyone to a party--in America, people just show-up, and it's bad form to complain about it.

Alexis de Tocqueville mentions this feature of America several times in his book Democracy in America, I think especially in volume 2.